1. Field of the Invention
The present invention relates to a vehicle and a method for controlling engine start in a vehicle.
2. Background Art
With the increasing concern for reducing vehicle emissions, hybrid electric vehicles (HEV's) are becoming increasingly popular. One way that an HEV provides reduced emissions, is by shutting off the engine and operating the vehicle with one or more electric motors. The engine is usually shut off during times when vehicle torque requirements are not high. If, for example, the driver demands a torque increase beyond a certain point, the engine may need to be restarted to augment the torque produced by the motor. Thus, while the vehicle is operating, the engine may be started and stopped many times, depending on the operating conditions.
In addition to providing torque for the vehicle wheels, an electric motor in an HEV can be used to start the engine. The motor, which receives power from an electrical storage device, such as a battery, spins the engine until a predetermined speed is reached, whereat fuel is provided to the engine, and the engine begins to produce torque. Because the engine may be stopped and started many times while the vehicle is operating, the engine start should be transparent to the driver. Therefore, the engine should be started such that the amount of torque delivered to the wheels will not be reduced because the state of charge (SOC) of the battery is too low. If the SOC is allowed to drop below a certain point, the amount of torque provided to the vehicle wheels by the motor may need to be reduced in order to provide enough torque to start the engine. This would create a non-transparent engine start, and degrade drivability.
One attempt to deal with this issue is discussed in U.S. Pat. No. 6,424,157 issued to Gollomp et al. on Jul. 23, 2002. Gollomp et al. describes a system and method for monitoring a vehicle battery, including a battery used in an HEV. In the Gollomp et al. system, a computer is preprogrammed with battery performance data taken from external sources. Ambient temperature data is then collected, and maximum current use data for starting current versus measured ambient temperature is calculated. A message can then be provided to a vehicle operator to indicate the amount of time remaining before the battery will not have enough power to start the engine. If the time remaining is less than some predetermined amount of time, the message may appear in the form of a warning.
One limitation of the Gollomp et al. system is that it only provides an indicator of when the battery can no longer start the engine; it does not provide an indicator of when starting the engine will reduce the torque provided to the vehicle wheels. The point at which torque to the vehicle wheels must be reduced may come well before the point at which the battery can no longer provide enough power to start the engine. Therefore, the Gollomp et al. system does not ensure a transparent engine start.